[Medial canthal anatomy and the lacrimal drainage system]

The Lacrimal drainage system has a close relationship with the orbicularis oculi muscle. The lacrimal sac and the lacrimal canaliculus are divided into 2 compartments by Horner's muscle. Namely, the lacrimal sac has one part attached to Horner's muscle and one part unattached, and the lacrimal canaliculus has one part in Horner's muscle and a part out side. The upper part of the sac dilates during contraction of Horner's muscle and is compressed during muscle relaxation. On the other hand, the lower part of the sac, affected by the orbital pressure, is compressed during contraction of the orbicularis oculi muscle, but dilates during relaxation. The lacrimal canaliculus in Horner's muscle is compressed during contraction of Horner' s muscle, and dilates during muscle relaxation. However, the contraction of the Horner's muscle dilates the part of the canaliculus not covered by the muscle, but the muscle relaxation compresses the part between Horner's muscle and the medial canthal tendon. The movement of this part functions as a valve. In addition to this dynamic drainage, the architecture of the lacrimal sac and nasolacrimal duct aids the lacrimal drainage as a static drainage system.